From their site: Welcome to MOO. We're a printing company. Customers create a pack and then the pack is printed to pre-defined products. Create an XML document to determine where the user is placed in the process. For example, if you define a list of images that you want the user to select from to make a pack of cards, then they will be forwarded to the 'chooser' page. If you specify a list of images that you want them to actually use, and the 'crop' data associated with them then the user will be sent to the part of the process where they design the back of the cards.